Lead-acid storage batteries are produced in a variety of forms, with the differences in form being largely the result of compromises made between such factors as cost, weight, volume, capacity and service life. All lead-acid storage batteries have the common features of a positive plate in which the active material is lead dioxide, a negative plate composed of lead, and a water-sulfuric acid electrolyte.
In the manufacture of lead-acid batteries of the type in widest use, the positive plates are prepared from lead alloy grids filled with a paste principally containing lead oxide. The negative plates may also be prepared from lead grids filled with a paste similar to the positive paste. After the pasted plates are cured, the positive plates and the negative plates are assembled as cells in a battery case, the battery case is filled with electrolyte and the battery is subjected to a formation process. In the formation process, a charging potential is applied to the battery for a period of time to convert the lead oxide compounds of the positive plates to lead dioxide, the positive active material of the battery, and, if the negative plates are of the pasted type, to covert the lead oxide compounds of the negative plates to sponge lead, the negative active material of the battery. The cost of the energy required to perform the formation process is a significant part of the total cost of battery manufacture.
Various additives have been included in the lead oxide paste formulations in efforts to improve battery performance and life. To combat the tendency of the negative plate to contract in service, a small amount of "expander" such as lampblack or barium sulfate may be added to the negative paste.
U.S. Pat. No. 1,788,571, issued Feb. 2, 1927, for "Active Material for Storage Batteries" discloses a lead-acid battery in which the composition of the active material for the positive plate comprises about 100 pounds of lead dioxide, about 100 pounds of lead monoxide, about 4 pounds of tantalum oxide, about 2 pounds of molybdenum oxide and about 1 pound of powdered carbon. The tantalum oxide serves to absorb hydrogen and to strengthen the plate. No purpose for including powdered carbon in the composition is stated.
U.S. Pat. No. 4,631,241, issued Dec. 23, 1986, for "Retainer Type Lead-Acid Battery" discloses a battery in which the electrolyte is retained in a glass fiber mat. The opposed surfaces of the retainer mat must maintain intimate contact with the surfaces of the positive and negative plates for the battery for the full discharge capacity of the battery to be realized. To achieve such contact, the positive active material is composed of lead powder blended with anisotropic graphite powder in an amount of at least 0.1% of the weight of the lead powder. Upon formation of the battery, the graphite is said to oxidize to form an intercalation compound between graphite and acidic sulfate salt. This reaction causes the graphite to expand, thereby increasing the porosity of the plate and urging the plate into intimate contact with the electrolyte retainer mat.
U.S. Pat. No. 4,735,870, issued Apr. 5, 1988, for "Lead-Acid Battery Construction" discusses the process occurring during discharge of a lead-acid battery in which the lead dioxide of the positive active material, a fairly good conductor, is converted to lead sulfate, an insulator. The lead sulfate encapsulates the lead dioxide particles, limiting utilization to less than 50% of capacity, typically around 30%. Attempts to increase the conductivity of the positive paste by adding a conductive filler, such as graphite, are mentioned. However, it is concluded that graphite is not an effective additive for conductance enhancement in a lead-acid battery because the graphite is oxidized to acetic acid which combines with the lead ion, causing corrosion of the positive grid. The positive paste additive described and claimed in the '870 patent is tin dioxide dispersed in the paste as a powder or coated on a substrate for dispersion in the paste.
It is an object of the present invention to provide an additive for the paste from which the positive plate of a lead-acid battery is formed that will enhance the conductivity of the positive plate and thereby improve the performance of the battery.
Another object of the invention is to provide an additive for the paste from which the negative plate of a lead-acid battery is formed that will enhance the conductivity of the negative plate and thereby improve the performance of the battery, particularly during operation at low temperatures.
Another object of the invention is to provide a lead-acid battery in which the energy required for formation is substantially reduced from that required for formation of a conventional lead-acid battery of comparable capacity.
It is a further object of the invention to provide a lead-acid battery in which the amount of the theoretical capacity of the battery available for utilization is substantially increased over the capacity obtainable from prior lead-acid batteries.
Still another object of the invention is to provide a lead-acid battery having a reduced internal resistance as compared with a conventional lead-acid battery of similar size, weight and capacity.
Additional objects of the invention are to provide a lead-acid battery that may be rapidly recharged, that produces greater output for cold cranking loads, and that recovers rapidly after being highly discharged.
Other objects and advantages of the invention will become apparent as a fuller understanding of the invention is gained from the detailed description to follow.